A circuit board (sometimes referred to as a printed circuit board or a printed wiring board) is the basic building block for interconnecting electronic devices in a system. Electronic devices, usually integrated circuit (IC) devices, are mounted onto the circuit boards using a number of mounting mechanisms, such as by use of connectors or by directly mounting the devices onto a surface of the circuit board. A circuit board also includes the wiring required to interconnect the devices electrically, and the circuit board provides the primary support for the devices.
The density of signal lines in a circuit board is continuously increasing due to the increased density of circuits that can be formed on each IC chip. The number of input/output (I/O) pins that exist on each IC chip can be quite large, which means that a large number of signal wires are needed to carry signals from one IC chip to another component in the system. To increase the density of signal wires that can be provided in the circuit board, a circuit board is usually formed of multiple layers. Some layers contain signal wires for transmitting signals, while other layers contain power and ground reference planes, which are connected to ground or to a power supply voltage, e.g., a three-volt voltage, a five-volt voltage, or some other power supply voltage. In other arrangements of circuit boards, power reference planes are not used. To connect signal wires in different layers of the circuit board, vias are provided. A via is an electrical connection that is run through multiple layers of the circuit board to complete a signal path using different layers, or to provide an electrical connection to ground or power. Typically, the via is run generally perpendicularly to a main surface of the circuit board.
With large numbers of IC chips and signal wires in a circuit board, switching noise can be a problem during system operation, especially at high frequencies. To mitigate switching noise, surface mount technology (SMT) decoupling capacitors are commonly used. These capacitors are mounted to either the primary or secondary (top or bottom) surface of the circuit board, and connected to reference planes through vias. At high frequencies, a capacitor provides a low impedance bypass path for switching noise between a power supply voltage plane and a ground plane.
One issue associated with connecting decoupling capacitors to reference planes is the relatively high inductance resulting from the combination of the capacitor's package, vias, and the interconnecting structure from the decoupling capacitor to the vias. As frequencies increase into the hundreds of megahertz (MHz) or gigahertz (GHz) range, the impedance associated with the combined inductance of each decoupling capacitor circuit becomes much larger than the capacitive impedance associated with the decoupling capacitor itself. To reduce the package inductance, SMT capacitors are used. To reduce the interconnection inductance, low-inductance interconnections are used, such as short wires, short via holes, wide interconnects, multiple vias, and so forth. Nevertheless, because of the increased impedance caused by the inductance of the via between the decoupling capacitor and the reference plane, the capacitor may not be able to effectively provide a low-impedance bypass path for switching noise at high frequencies. In other words, because of a significant impedance introduced by the via inductance into the decoupling path, a capacitor loses its decoupling effectiveness in providing a bypass path for high frequency noise.
Another issue associated with circuit boards is radiated electromagnetic interference (EMI), which increases as signal density and switching speeds increase. EMI is a byproduct of noise currents generated by the switching of active devices in the circuit board. When noise currents reach the DC power bus (which includes power and ground reference planes) of the circuit board, noise voltages can be generated in the power bus that are proportional to the impedance of the power bus. The noise voltages can propagate throughout the power bus and radiate by various mechanisms.
In a circuit board in which active devices are switching at high rates (such as in the gigahertz range), or switching with rapid transitions (such as more rapid than a nanosecond), the noise currents generated in the power bus have more high-frequency spectral components, which may require the DC power bus of the circuit board to have a very low impedance to avoid problems at the high frequencies that are possibly in the spectrum of the noise currents. Decoupling capacitors are typically used to control the power bus impedance. However, as noted above, the effectiveness of conventional decoupling capacitors in maintaining a low power bus impedance is limited by frequency, since the parasitics of these capacitors may make them ineffective at frequencies of several hundred megahertz and above. As a result, the circuit board first resonates at a frequency determined by the parasitic inductance of the decoupling capacitors (referred to as lumped resonance) resulting in a peak in the impedance of the circuit board. At higher frequencies, the circuit board has distributed resonances, which are related to the physical dimensions of the circuit board.